The present invention relates to ellipsometers and in particular to small-spot spectroscopic ellipsometers.
In a conventional ellipsometer, the polarization is modulated by a rotating elementxe2x80x94either a polarizer or quarter wave platexe2x80x94placed either before or after the sample. On the other side lies a polarizer or waveplate polarizer combination. By measuring the signal at several different positions of the rotating component (a minimum of four), the ellipsometric parameters psi (xcexa8) and delta (xcex94), or more usually tan xcexa8 and cos xcex94, are determined. The ellipsometer may function at one wavelength or many wavelengths, in which case it is called a spectroscopic ellipsometer.
With a single wavelength only two parameters are measured and therefore only two unknowns can be determined, for example, thickness and index of a single layer film. With a spectroscopic ellipsometer, there are two independent parameters measured at each wavelength and many more sample unknowns can be determined. Typically, though, the unknowns are not completely independent with wavelength. Film thickness is constant versus wavelength, and refractive index obeys dispersion relationships that can be approximated by many different formulae. These formulae may have only a few independent coefficients to specify the index for several hundred or thousand wavelength data points.
A regression procedure is usually performed where the unknowns are varied to produce theoretical spectra of tan xcexa8 and cos xcex94, or xcexa8 and xcex94, or another set of ellipsometric parameters, alpha (xcex1) and beta (xcex2), that best matches the ones measured from the sample. Depending on the quality of the data and the correlation of the unknowns, the maximum number of unknowns that may be regressed is usually less than 8 or 10 even though there are usually between 265 and 2048 separate spectral data points. Many of these points are clearly redundant, a fact that is exploited by the present invention.
One of the biggest problems in constructing any ellipsometer is the rotating mechanical components which must be very precise and: are relatively large since the light goes through the center shaft. Both the size and complexity are big disadvantages for integrated ellipsometers which must be compact and relatively inexpensive. An integrated instrument is one that is located within or attached to a process tool, such as a semiconductor wafer resist development track, that is helping to produce the samples which are to be measured.
Another problem for small spot spectroscopic ellipsometers is the limited light that is available. When a laser can not be used for the light source, there are fundamental limits to the brightness with which a small spot on the sample can be illuminated. Usually the signal for a small spot spectroscopic ellipsometer must integrated over some fraction of the period of the rotating element, and then a few turns are averaged together.
There have been other designs for ellipsometers with no moving parts, but none that have been ideal for a small-spot, integrated, spectroscopic ellipsometer. One design uses an acousto-optic modulator in place of the rotating element, but these vibrate at a high frequency meaning that it is impossible to integrate the signal for very long. These also do not work well over a very broad spectral range.
Another design directs light from different portions of the beam aperture to different analyzers and detectors where the analyzers are mounted at different fixed angles. For a spectroscopic ellipsometer, this design would either require multiple spectrometers and be very expensive, or a monochromator in the illumination and be very slow. The light level on each detector would also be reduced. In a small spot ellipsometer, breaking the aperture into separate beams also increases the diffraction limited spot size on the water.
Another design uses arrangements of polarizing beamsplitters and waveplates to send differently polarized beams to different detectors. Again, this scheme would require multiple spectrometers or a monochromator if it is to be spectroscopic. The wavelength range is also limited due to the imperfectly achromatic waveplates.
An object of this invention is to provide a small-spot, spectroscopic ellipsometer with no rotating elements that is suitable for an integrated instrument. The wavelength range can encompass the UV through near IR. The VUV and far IR ranges may be measured with a different choice of components.
The basic concept of the invention is that the polarization is modulated with respect to wavelength instead of time as in a conventional ellipsometer.
The spectroscopic ellipsometry instrument includes (1) a multiwavelength light source providing a light, beam in an illumination optical path directed toward a sample surface under examination, (2) a spectrometer receiving light through an aperture in a collection optical path from the sample surface and providing a spectroscopic measurement signal to an ellipsometry data processor, (3) optionally, at least one objective in the illumination and collection paths focusing the light beam and gathering light from a small spot (e.g. a size less than 500 xcexcm) on the sample surface, (4) a polarizer and polarization analyzer of known orientation in the illumination and collection paths, and (5) a stationary polarization modulator in at least one of the illumination and collection optical paths, wherein the modulator modulates the light polarization cyclically (e.g., pseudo-periodically or sinusoidally) versus wavelength, the polarization being modulated by at least one-half period over a wavelength range of the instrument. Several ellipsometer embodiments are described in further detail below, differing in using oblique or perpendicular incidence on the sample, using completely separate illumination and collection light paths with two objectives and separate polarizer and polarization analyzing elements or using a beamsplitter with a portion of the illumination and collection paths between the beamsplitter and sample being in common and with a single common objective, and also differing in the placement of the modulator. This polarization-versus-wavelength modulation concept may be applied to large-spot spectroscopic ellipsometers as well.